Axinite-(Mg)

Axinite-(Mg) is a borosilicate mineral of aluminum, calcium and magnesium of the axinite group, with magnesium as the dominant cation in the place of the structure that can also be occupied by iron and manganese. It was discovered in gem material from Merelani Hills, Lelatema Mts, Manyara Region, Tanzania, which is consequently its type locality.^[3] It was initially called magnesioaxinite, referring to its membership in the axinite group and the role of magnesium as the dominant cation. The International Mineralogical Association (IMA) later changed its name to axinite-(Mg).^[4] Occasionally it has been carved as a collection gem.

Axinite-(Mg)
Crystal fragment of gem axinite-(Mg) from Merelani Hills,Manyara, Tanzania
General
Category	Silicate mineral
Formula (repeating unit)	Ca2MgAl2BSi4O15OH
Crystal system	Triclinic
Identification
Color	Light blue, pink
Crystal habit	Crystals with the characteristic axe shape
Cleavage	Good {100}
Fracture	Uneven to subconchoidal
Tenacity	Somewhat brittle
Mohs scale hardness	6+1⁄2 - 7
Luster	Vitreous
Streak	White
Diaphaneity	Transparent or translucent
Specific gravity	4.97
References	^[1]^[2]

Physical and chemical properties

Like the rest of the minerals in the axinite group, axinite-(Mg) belongs to the triclinic system, appearing in the form of crystals with the characteristic ax-shaped morphology. Its structure can be described as a sequence of alternating layers of cations coordinated tetrahedrally and octahedrally. ^[5]

Deposits

The axinite group minerals are found in medium-to-low contact metamorphism, regional or metasomatic environments, in boron-containing environments. Axinite-(Mg) appears more frequently in areas of contact metamorphism.

It is a relatively rare mineral, known in about a dozen locations in the world. In addition to the type locality, already indicated, in which specimens with transparent crystals of various colors, up to 3 cm in size, have been found in the area of Lunning, Mineral Co., Nevada (USA), as violet brown crystals.^[6] In Spain, axinite-(Mg) associated with crystalline calcite has been found in the diabase of a quarry located in El Zurcido, Adamuz (Córdoba) .^[7]

References

1. Handbook of Mineralogy
2. "Axinite-(Mg): Axinite-(Mg) mineral information and data". Mindat.org. 2019-08-05. Retrieved 2019-08-05.
3. Jobbins, E.A., Tresham, A.E. y Young, B. (1975). "Magnesioaxinite, a new mineral found as a blue gemstone from Tanzania". Journal of Gemmology. 14 (8): 368–375. doi:10.15506/JoG.1975.14.8.368.{{cite journal}}: CS1 maint: multiple names: authors list (link)
4. Burke, Arnest A.J. (2008). "Tidying up mineral namesː an IMA-CNMCN scheme for suffixes, hyphens and diacritical marks". The Mineralogical Record. 39: 131–135.
5. Takéuchi, Y., Ozawa, Y., Ito, T., Araki, T., Zoltai, T. y Finney, J.J. (1974). "The B2Si8O30 groups of tetrahedra in axinite and comments on deformation of Si tetrahedra in silicates". Zeitschrift für Kristallographie. 140 (5–6): 289–312. Bibcode:1974ZK....140..289T. doi:10.1524/zkri.1974.140.5-6.289.{{cite journal}}: CS1 maint: multiple names: authors list (link)
6. Jobbins, E.A., Tresham, A.E. y Young, B.R (1980). "Magnesioaxinite from Lunning, Nevada, and some nomenclature designations for the axinite group". The Mineralogical Record. 11: 13–15.{{cite journal}}: CS1 maint: multiple names: authors list (link)
7. Calvo, Miguel (2018). Minerales y Minas de España. Vol. IX. Silicatos. Madrid, Spain: Escuela Técnica Superior de Ingenieros de Minas de Madrid. Fundación Gómez Pardo. p. 175. ISBN 978-84-8321-883-9.